Rechargeable electrochemical cells are designed for use in applications which are light-weight and portable, require long operating life, and are incompatible with requirements of periodic maintenance. They may be used as direct replacements for primary double AA, C and D cells in numerous consumer devices such as calculators, portable radios and flashlights. They are more often integrated into the form of a sealed power pack designed to interface with or be an integral part of a specific device.
The rechargeable electrochemical cell is ideally suited to serve as a portable power source due to its small size, light-weight, high power capacity and long operating life. The rechargeable cell is an "install and forget" power source. With the exception of periodic charging, a rechargeable cell will perform without attention, rarely becoming the limiting factor in the life of the device it powers.
Secondary cells using a rechargeable hydrogen storage negative electrode are known in the art. See, for example, U.S. Pat. Ser. No. 4,551,400 for HYDROGEN STORAGE MATERIALS AND METHODS OF SIZING AND PREPARING THE SAME FOR ELECTROCHEMICAL APPLICATIONS the disclosure of which is incorporated herein by reference. Hydrogen storage cells operate in a different manner from lead-acid, nickel-cadmium or other prior art battery systems. Hydrogen storage electrochemical cells utilize a metal hydride negative electrode that is capable of reversibly electrochemically storing hydrogen. In one exemplification the cell employs a positive electrode of nickel hydroxide material, although other positive electrode materials may be used. The negative and positive electrodes are spaced apart in an alkaline electrolyte, and may include a suitable separator, spacer, or membrane therebetween.
Upon application of an electrical current to the negative electrode, the negative electrode material (M) is charged by the electrolyte decomposition of water to form the hydride and an OH-ion: EQU M+H.sub.2 O+e.sup.- .fwdarw.M-H+OH.sup.- (Charging)
Upon discharge, the hydride is decomposed to release hydrogen within the cell, reacting with OH-ion to form water, and releasing an electron to the external circuit to provide an electric current: EQU M-H+OH.sup.-.fwdarw.M+H.sub.2 O+e.sup.- (Discharging)
The negative electrode reactions are reversible.
The reactions that take place at the positive electrode are also reversible. For example, the reactions at a conventional nickel hydroxide positive electrode as utilized in a hydrogen rechargeable secondary cell or battery are: EQU Ni(OH).sub.2 +OH.sup.-.fwdarw.NiOOH+H.sub.2 O+e.sup.- (Charging), and EQU NiOOH+H.sub.2 O+e.sup.-.fwdarw.Ni(OH).sub.2 +OH.sup.- (Discharging).
Hydrogen storage negative electrode cells are designed to operate with a nickel hydroxide positive electrode and a hydrogen storage alloy negative electrode, separated by non-woven, felted, nylon or polypropylene separator. The electrolyte is generally an alkaline electrolyte, for example, 20 to 40 weight percent potassium hydroxide, where lithium hydroxide may also be present in limited quantity.
A cell utilizing an electrochemically rechargeable hydrogen storage negative electrode offers important advantages over conventional secondary cells. Rechargeable hydrogen storage negative electrodes offer significantly higher specific charge capacities (ampere hours per unit mass and ampere hours per unit volume) than do either lead negative electrodes or cadmium negative electrodes. As a result of the higher specific charge capacities, a higher energy density (in watt hours per unit mass or watt hours per unit volume) is possible with hydrogen storage batteries than with the prior art systems, making hydrogen storage cells particularly suitable for many commercial applications.
Hydrogen storage cells are of two types, sealed cells and vented cells. In addition to differences in their basic construction implied by nomenclature, the two types of cells differ in their modes of operation. During normal operation, a sealed cell does not permit the venting of gas to the atmosphere. By way of contrast, in a vented cell, venting may be part of the normal operating behavior. As a result of this difference the vent assemblies associated with each type of cell are quite different from each other, and the amounts of electrolyte in the cell container relative to the electrode geometry are significantly different.
Sealed cells are generally manufactured in many configurations, predominantly including cylindrical and rectangular. Sealed cells are designed to operate in a starved electrolyte configuration. That is sealed cells are designed to operate with a minimum amount of electrolyte. The cell enclosure for a sealed cell is normally a metal enclosure designed for operation at a typical cell operating pressure that can be of up to about 100 pounds per square inch absolute or even higher. Sealed cells are characterized by the substantial absence of any required maintenance, and even one time venting cells require some periodic maintenance.
By way of comparison, vented cells, which have a nickel hydroxide positive electrode, and a hydrogen storage alloy negative electrode, typically have a woven or non-woven separator, e.g., of nylon or polypropylene. The vented cell differs most strongly from the sealed cell in that it is operated in a flooded condition. By a flooded condition is meant that the electrodes are completely immersed in, covered by, and wetted by the electrolyte. Thus, such cells are sometimes referred to as "flooded cells." A vented cell is further differentiated from a sealed cell in that the vented cell is designed for normal operating pressures of only up to about 25 pounds per square inch, after which excess pressures are relieved by a vent mechanism.
The discharge capacity of the nickel positive electrode is limited by the amount of electrolytes the amount of active material and charging efficiencies. The charge capacity of the negative, hydrogen storage alloy electrode is limited by the amount of active material used since its charge efficiency is nearly 100 percent until a nearly full state of charge is reached. To maintain the fullest capacity of the hydrogen storage electrode, adequate precautions must be taken to avoid oxygen rcombination or hydrogen evolution before full charge is accomplished. This is generally accomplished by providing an excess of negative electrode material. However, precautions must be taken in design and fabrication of sealed cells to avoid affects of over-pressurization associated with overcharge at dangerously high charge rates.
Rechargeable sealed cylindrical cells use a cylindrical nickel-plated steel case as the negative terminal and the cell cover as the positive terminal. The electrodes, are wound to form a compact "jelly roll" with the electrodes of opposite polarity isolated from each other by a porous separator. An insulator separates the positive cover from the negative cell can.
Conventional, cylindrical, rechargeable cell construction relies upon a tab which is extended from one point on each plate. This creates a single current path through which current must be distributed to the entire electrode area during charging and discharging.
While rechargeable cells have many useful applications, there are many instances in which damage may occur to the cells of the type described hereinabove. For instance, during periods of overcharging for extended lengths of time, excess pressure can be generated inside the cell can of the rechargeable cell. As the pressure of these gases increases, so also does the danger of an explosive failure. Failure, e.g., explosive failure, may occur at that point in time in which the internal pressure of gases surpasses the pressure which the cylindrical cell can is capable of withstanding.
Cylindrical, rechargeable cells of the prior art included a "one time only" venting mechanism where for example, a rupturable diaphragm and blade apparatus was employed. As internal cell pressure increased, the blade was forced against the diaphragm. As the pressure further increased, the blade punctured the diaphragm, allowing excess gases to escape through the ruptured diaphragm. This destructive type of venting mechanism was both unpredictable from batch to batch and from cell to cell within a batch. Moreover, destructive venting is good for only one excess pressure situation. After the diaphragm is punctured it cannot even hold normal cell operating pressures. In one exemplification, the diaphragm is punctured by an upward driven plunger. In another exemplification the diaphragm is forced upward towards the incising blade. As the internal cell pressure reaches the critical level, i.e., as the diaphragm reaches a critical deformation, the diaphragm is forced against the incising blade which would thus puncture the diaphragm, allowing the escape of accumulated cell pressure. An example of this type of device is fully disclosed in U.S. Pat. No. 3,415,690 incorporated herein by reference.
One shortcoming present in this and other methods of venting rechargeable electrochemical cells is that the venting mechanism present thereon was a "one time only" mechanism. Once open, the electrolyte material inside said rechargeable cell is exposed to the surrounding atmosphere. The electrolyte levels would be disturbed and thus the ability of the cell to retain and dispense an electrochemical charge would be deleteriously effected. Another shortcoming was the dependence of venting on the movement of a thin, deformable diaphragm against the incising tooth.
Other prior art venting assemblies included square or polygonal shaped rubber stoppers, attached to a vented plate at three of the square's four corners. This assembly proved beneficial in that it was not a "one time only" mechanism. Unfortunately, this assembly either failed to reliably vent at a given internal cell pressure or "stuck" open thereby causing the evaporation of the electrolyte material. This failure is attributable to the formation of a "pocket" by the stopper which prevented cell gasses from properly venting. The result, of course, was failure of the rechargeable cell. An example of this type of ventable cover assembly is fully disclosed in U.S. Pat. No. 3,994,749, incoporated herein by reference.
In addition to the shortcomings inherent in the vent plugs of the prior art, cell cover plates of the prior art were also deficient in their design. Specifically, cell cover plates of the prior art lacked the ability to exert a uniform outward force in response to radial compression. This inability required excessive pressures be applied to said plates to effect an air-tight seal between the cover plate and the cell can. This excessive pressure often exceeded the shatter point of the cover plate, the insulating ring or both, thus causing failure of the seal. Typical cover assemblies lacking the ability to exert responsive force are disclosed, for example, in U.S. Pat. Nos. 3,986,083 and 4,271,241 both of which are incorporated herein by reference.
There is therefore a need to develop an easily sealable cover assembly including a venting mechanism whereby excessive internal cell pressures can be vented off or released from the rechargeable cells at predictable pressures without destruction of the cell, as by permanently exposing the electrolyte inside the electrochemical cell to surrounding ambient conditions. Especially important is the requirement that in a production run of many cells, the venting mechanism be capable of repeatably venting at a uniform internal cell pressure.